1. Field of the Invention
The present invention concerns ceramic substrates which are sintered at low temperatures and are used as circuit substrates in electronic devices.
2. Description of the Prior Art
Alumina substrates generally in use in electronic elements are sintered at high temperatures, e.g., 1500.degree. to 1600.degree. C. However, low temperature sintering has recently been developed for ceramic substrates used to mount electronic parts. Further, for attaining a multi-layered alumina circuit substrate, a conductor interposed between layers is limited to a high melting point material having relatively high resistivity such as tungsten or molybdenum. As such, a reducing atmosphere Is necessary when sintering for preventing the material from oxidizing. As a result, the resistance of the wirings is increased, resulting in restrictions in the wiring pattern when transmission loss of electric signals is taken into consideration. That is, a great amount of energy has to be consumed, in the case of using alumina, when the substrate is sintered for forming a multi-layered conductor circuit, and this remarkably increases the production cost and also imposes a large restriction in characteristics. In order to overcome such a drawback, it has become necessary to develop ceramic materials which are sinterable at a temperature of 800.degree. to 1000.degree. C., preferably 850.degree. to 900.degree. C., and to develop a multi-layered structure therefor. Lowering of the sintering temperature is advantageous in enabling the use of conductors of low resistance value such as Au, Ag--Pd, Ag and Cu, which are low temperature sinterable materials.
However, although known substrates sintered at low temperature are sinterable at 800.degree. C. to 1000.degree. C., their mechanical strength is considerably reduced as compared with a strength of 3000 to 4000 kgf/cm.sup.2 of alumina substrates.
For instance, Japanese Patent Publication HEI 3-53269 discloses a substrate having a mechanical strength of about 2,000 kgf/cm.sup.2 obtained by adding an Al.sub.2 O.sub.3 powder to CaO--SiO.sub.2 --Al.sub.2 O.sub.3 system glass, sintering the mixture at 800.degree. C. to 1,000.degree. C., and depositing anorthite crystals. However, its mechanical strength is inferior to that of an alumina substrate.
Further, Japanese Patent Laid-Open Hei 3-33026 discloses a composition sinterable at 850.degree. C. obtained from a combination of crystallized glass in which ZnO and alkaline earth metal oxides contribute to crystallization and a ceramic filler, but it mentions nothing about the strength.